Next generation mobile communication and wireless transmission systems require improved data rate and system capacity needed in a multi-cell environment. Accordingly, the multi-cell environment adopts a multi-node or multi-cell cooperative scheme in order to obtain maximum efficiency from limited resources. Such a cooperative scheme has vastly superior performance to a non-cooperative scheme in which respective nodes do not cooperate with each other by operating as independent base stations (BSs) (Node-Bs (NBs), eNode-Bs (eNBs), access points (APs), etc.).
To meet such demand, research into multiple input multiple output (MIMO) systems for transmitting data using multiple antennas is ongoing. Among MIMO systems, a closed-loop MIMO system using channel information to improve data transmission rate in multi-cell environments enhances transmission performance using the channel information. Moreover, to use a proper codebook according to each cell environment in the multi-cell environment, multiple codebooks are used to provide improved service to a UE.
Generally, in a MIMO system, the UE may discern information about a downlink channel, which is a reception channel, using received data. However, it is difficult for a BS to know the downlink channel information. Therefore, in order to improve system performance using the channel information, the BS should discern the channel information. When a time division duplex (TDD) scheme is used, since uplink/downlink channels between the BS and the UE are identical, the BS can be aware of the channel information. In the closed-loop MIMO system in the multi-cell environment, a BS allocates an optimal pre-codebook to UEs based on a channel situation between the BS and each UE in order to provide a high-quality service to the UEs in a cell.
The closed-loop MIMO system transmits data using information about transmission channels of the UEs served by the BS. In this case, since the BS does not know the information about the transmission channels of the UEs served thereby, the BS is given feedback on the channel information, for example, channel state information (CSI), a channel quality indicator (CQI), a preferred matrix index or precoding matrix index (PMI), etc., from the UEs.
In the multi-cell environment, the UE estimates a channel over which data has been transmitted using a signal received from the BS. Next, the UE calculates a CQI using the estimated channel. The CQI is used to apply a Modulation and Coding Scheme (MCS) suitable for a channel situation when the BS transmits data. Using the estimated channel and a plurality of codebooks, a channel coefficient most suitable to channel state, i.e. a precoding vector of a codebook, may be selected from a codebook used in each cell. Channel information obtained using the channel estimated by the UE is transmitted through a feedback channel between the BS and the UE. The BS selects an MCS and a precoding vector of the codebook using the channel information received from the UE. Next, the BS transmits data to the UE using the selected MCS and the selected precoding vector of the codebook.
A wireless communication system has evolved to a multi-node system having multiple accessible nodes in the vicinity of a user. The multi-node system may have higher system performance by cooperation between nodes. For example, the case in which transmission/reception of each node is managed by one control station and each node operates like an antenna (or an antenna group) for one cell exhibits much better system performance than the case in which respective nodes do not cooperate with each other by operating as independent BSs (advanced BSs (ABSs), NBs, eNBs, APs, etc.). In this case, the multi-node system may be referred to as a distributed antenna system (DAS).
The multi-node system may also operate as a multi-cell system in which each node has an independent physical identifier (ID). If the multi-cell system is configured such that coverage of respective nodes is overlaid, this is referred to as a multi-tier network.
As an element capable of configuring a node, there is a remote radio head (RRH). The node may be referred to as a point. As shown in the above example, nodes in the multi-node system may have the same physical cell ID or respective physical cell IDs.
As node density increases, interference between nodes may be severely generated in the multi-node system. For example, if a node (BS or eNB) supporting only a closed subscriber group (CSG) UE (user or MS), (generally, a CSG femto-BS or HeNB), is used in the multi-tier network, the CSG node may create very strong interference with respect to UEs except for CSG UEs present in corresponding node coverage. That is, a UE which is not served by a first node may be subjected to interference by data transmitted to a UE which is served by the first node.
In such a multi-node system, an effective method for eliminating an interference signal generated by a neighboring node is needed.